Mirrors are optical apparatuses built to reflect, concentrate and/or focus electromagnetic radiation impinging on their surfaces.
Various multilayer mirrors have been developed which are presented, for example, in U. S. Patent Application Publication No. 2006/0141042, U.S. Pat. No. 8,034,263, U. S. Patent Application Publication Nos. 2009/0148095, 2010/0033702, 2012/0162772, 2012/0170012. The mirrors presented in these references are intended either for terrestrial applications or for outer space applications.
For example, U. S. Patent Application Publication No. 2012/0162772 presents that specifically for extra-terrestrial applications, mirrors must withstand harsh environments (such as UV radiation, temperature cycling, temperature gradients, contamination, self-contamination, and the impact of particles impinging on the optical surface). Additional protection buffering layers, as well as layers to balance the coefficient of thermal expansion of the mirror, may be used for extra-terrestrial mirror applications.
U. S. Patent Application Publication No. 2009/0148095 describes electrically and/or magnetically steered arrays using nanowires (formed from a variety of materials including gallium nitride, silicon, silicon germanium, zinc oxide, lead zirconate, titanate, cadmium sulfide, indium phosphide and others). When nanowires are grown, the space therebetween and around is filled, to form a rigid nanowire waveguide structure. The nanowires within the mirror serve as waveguides for optical frequency radiation. By controlling the nanowire waveguide array through illumination with a phase coherent source (such as a laser), a phase delay in the optical frequency beam may be generated at the output of the nanowire waveguide arrays causing the optical frequency beam to deflect upon exiting. Thus, steering of the optical frequency beam may be attained by increasing or decreasing the phase delay of the output optical frequency beam.
Historically, dating from the time of Sir Isaac Newton, telescope making has emphasized rigidity. This meant that, as apertures have increased, the mirrors have become thicker and more massive. This tendency has been carried out to the point where it was no longer effective due to cost, mass, and thermal response times.
Since the 1980s, a new approach has evolved based on the concept of an “active” optics, which is adapted for new generation of telescopes using thin, lighter weight mirrors. Such mirrors are too thin to maintain themselves rigidly in the correct shape. Active optics is a technology used with reflecting telescopes, which actively shapes telescope mirrors to prevent deformation due to external influences such as wind, temperature and/or mechanical stress. Active optics compensates for distorting forces, that change relatively slowly, and roughly on timescales of seconds. The telescope is therefore actively adjusted to keep its optimal shape. In an active system, external sensors and actuators are attached to the back of the mirror to apply dynamic forces to the mirror body to bend or deform the optical surface so as to counteract the effects of gravity, thermal distortion and wind buffering.
Active optics uses a combination of actuators, an image quality detector, and a computer to control the actuators to obtain the best possible image.
One of the tradeoffs in the requirement for a control system employing a large number of external sensors and actuators attached to the mirrors, is the associated high density of wiring harnesses and connectors, and the need for bulky and complex reaction structures.
The modern telescopes use also adaptive optics, which operates on a much shorter timescale (than the active optics) to compensate for atmospheric effects, rather than for mirror deformation. The adaptive optics corrects image distortions due to the atmospheric turbulence using a wavefront sensor measuring the distortions introduced by the atmosphere on the timescale of a few milliseconds. A computer calculates the optimal mirror shape to correct the distortions, and the surface of the deformable mirror is reshaped accordingly.
A deformable mirror (DM) corrects incoming light so that the images appear sharp. The DM usually has many degrees of freedom. Typically, the degrees of freedom are associated with the mechanical actuators. The number of actuators determines the number of degrees of freedom (wavefront inflections) the mirror can correct. Deformable mirrors with large actuator pitch and large numbers of actuators are bulky and expensive.
Numerious concepts of actively and adaptively controlled mirrors have been proposed to compensate for unwanted distortions and effects of gravity, wind, etc. For example, segmented mirrors may be formed by independent flat mirror segments. Each segment can move a small distance back and forth to approximate the average value of the wavefront over the patch area. Unfortunately, these mirrors require a large number of actuators. This concept was used for fabrication of large segmented primary mirrors for the Keck telescopes, JWST, and the future E-ELT. An accurate co-phasing of the segments and reduction of the diffraction patterns introduced by the segment shapes and gaps adds to the complexity and bulkiness of the system.
In addition, continuous faceplate mirrors with discrete actuators may be formed by the front surface of a thin deformable membrane. The shape of the plate is controlled by a number of discrete actuators that are fixed to its back side.
Additionally, magnetics concept mirrors may be fabricated which are based on continuous reflective surface motioned by magnetics actuators.
Furthermore, membrane concept mirrors may be formed by a thin conductive and reflective membrane stretched over a solid flat frame. The membrane can be deformed electrostatically by applying control voltages to electrostatic electrode actuators that can be positioned under or over the membrane.
Ferrofluid concept mirrors have been suggested which are liquid deformable mirrors made with a suspension of small (about 10 nm in diameter) ferromagnetic nanoparticles dispersed in a liquid carrier. In the presence of an external magnetic field, the ferromagnetic particles align with the field, the liquid becomes magnetized and its surface acquires a shape governed by the equilibrium between the magnetic, gravitational and surface tension forces. Using proper magnetic field geometries, any desired shape can be produced at the surface of the ferrofluid. This concept offers a potential alternative for low-cost, high stroke and large number of actuators deformable mirrors.
Despite the progress in the active and adaptive optics, most of the optic systems still use a complicated, bulky and a heavy weight structure requiring an expanded array of external sensors and actuators, and a bulky reaction support, as shown in FIG. 1, to flex the mirrors to control their optical configuration to compensate for unwanted external influences and optical distortions.
In 2008, Dr. Peter C. Chen, astrophysicist of NASA Goddard Space Flight Center, and the inventor of the present system and method, suggested (in http://science.nasa.gov/science-news/science-at-nasa/2008/09jul_moonscope) a concept of using lunar dust to fabricate (directly on the Moon) highly economical telescope mirrors from composite materials, i.e. the synthetic materials made by mixing fibers or granules of various materials into epoxy, and letting the mixture harden. Composites are ultra-lightweight materials which possess extraordinary strength. It was suggested to mix epoxy and a small quantity of carbon nanotubes for fabrication of the telescope mirror. A small telescope mirror was made using a long known technique called spin-casting. The carbon nanotubes make the composite material a conductor. Conductivity allows a large lunar telescope mirror to reach thermal equilibrium quickly with a monthly cycle of lunar night and day. Dr. Chen also suggested that the conductivity would also allow astronomers to apply an electric current as needed through electrodes attached to the back of the mirror to maintain the mirror's parabolic shape against the pull of lunar gravity as the large telescope was tilted from one portion of the sky to another.
Although suggesting the fabrication of telescope mirrors using carbon nanotube/epoxy which would be uniquely suited for deformable mirrors applications through external stimulus (such as electric current), the fabrication technique suggested was limited to the spin-casting technology, and only to the carbon nanotube/epoxy as a material for the telescope mirror. The publication suggests neither other possible techniques, nor provides specifics of composite materials telescope technologies.
It would be desirable to provide various polymer matrix composite materials suitable for application in deformable “smart” mirrors, as well as various fabrication techniques for manufacturing of flat, as well as curved mirrors with high degree of uniformity which may be actively controlled by external actuation stimuli responsive to internally sensed conditions.